The marine environment has proven to be a very rich source of extremely potent compounds that have demonstrated significant activities in anti-tumor, anti-inflammatory, analgesia, immuno-modulation, allergy and anti-viral assays. Although the case can and has been made that the nucleosides such as Ara-A and Ara-C are derived from knowledge gained from investigations of bioactive marine nucleosides, no drug directly from marine sources (whether isolated or by total synthesis) has yet made it to the commercial sector in any human disease. However, as shown in this review, there are now significant numbers of very interesting molecules that have come from marine sources, or have been synthesized as a result of knowledge gained from a prototypical compound, that are either in or approaching Phase III clinical trials in cancer, analgesia and allergy, with a very substantial number of other, quite different potential agents following in their wake, in these and in other diseases.